PVC Pneumatic Cylinder

guss

I am in a robotics competition where we can only use certain parts. My team built a scissor lift, and we are currently looking for a better way to lift it. It takes upwards of 100 pounds of force to lift, and when it's down its base is about 3" by 10".

I want to build a pneumatic cylinder, but it will be difficult. The rules state:
We can also use PVC cement and surgical tubing.

Does anyone have an idea of how to do this? Especially difficult will be making a sliding mechanism that doesn't leak, compressing the cylinder, and getting it to create so much force will be especially difficult.

Right now, I have at least a rough idea of how to make the cylinder, but no idea how to compress it. We cannot use regular compressed air cylinders in the competition, but I suppose I could store air in a sealed PVC pipe or build a compressor to put on the robot.

Thanks for your help!

Q_Goest

Hi guss. Are you allowed to use O-rings? What is it about the design of the pneumatic cylinder you find difficult? For a robot, I would assume they allow you to use off the shelf solenoid valves and controls?

NeuronsAtWork

Hello, Guss--

I do not mean to rain on your parade, but you need to be aware that the use of PVC pipe to store or transport compressed air or gases is forbidden by OSHA and a number of state codes, as well as pipe manufacturers.

Here is one of many releases on the subject:

from: http://torque1st.clubfte.com/OSHA_PVC_Pipe.htm
--------------------------------------------------------

STATE OF WASHINGTON
Department of
Labor & Industries
Hazard Alert

For more information, call: 1-800-423-7233

FOR IMMEDIATE RELEASE May 26, 1988

PVC pipe not to be used in compressed air systems

OLYMPIA -- The Department of Labor and Industries warned today that plastic
polyvinyl chloride (PVC) pipe cannot be used in compressed air piping systems
without the risk of explosion.

When PVC piping explodes, plastic shrapnel pieces are thrown in all directions.

"We're seeing more incidents of explosive failure, and we're citing more employers
for using PVC air system piping," said Paul Merrill, senior safety inspector in
L&I's Spokane office.

"It's probably just a matter of time before someone gets seriously injured in one of
these explosions unless everyone pays more attention to the manufacturer's
warnings," Merrill said.

Last year, a section of PVC pipe being used for compressed air exploded 27 feet
above a warehouse floor. A fragment of the pipe flew 60 feet and embedded itself in
a roll of paper. Fortunately, nobody was in the area at the time.

A PVC pipe explosion in a new plant in Selah broke an employee's nose and cut his
face.

PVC piping buried 3 feet underground at a Yakima manufacturing plant exploded,
opening up a crater approximately 4 feet deep by 3 feet across.

Only one type of plastic pipe has been approved for use with compressed air. That
pipe, Acrylonitrile-Butadiene-Styrene (ABS), is marked on the pipe as approved for
compressed air supply.

By law, employers must protect their workers by avoiding the use of unapproved PVC
pipe in such systems. Existing compressed air systems which use PVC piping must be
completely enclosed, buried or adequately guarded according to specifications
approved by a professional consulting engineer.

NOTICE TO EMPLOYERS: If you have questions about the suitability of a material for
air system piping, call Labor and Industries at the number listed above for a free
consultation.

NOTICE TO EMPLOYEES: If you suspect that a pressurized PVC piping hazard exists,
bring it to the attention of your employer. If you do not obtain satisfactory
results, you may file a confidential complaint with the Department of Labor and
Industries. Complaints are investigated promptly.

--------------------------------------------------------

You risk seriously injuring yourself and others doing this, so I would strongly recommend a different approach, and also bringing this to the attention of whomever is sponsoring the competition and writing the rules. Please note that I have been a facility engineer for 35 years and have seen it all at one point or another, so am speaking with some level of expertise on the subject. Please be careful!

guss

No, we can't use those.

Okay, thanks for your advice. If we did this, I think a hydraulic cylinder would be better, anyway. I am also leaning toward using a threaded screw and bolt. I am going to build both a threaded screw/bolt setup and a hydraulic cylinder setup and see which one works best.

LJW

Hi,

To reach 100lb of force from a 3" sch.40 cylinder you'll only need about 13.5 psi which I assure you is very safe, especially in pipe pressure rated for potable water (often upwards of 100 psi). You could go to a fluid filled setup if you still have the safety concerns but it will be much heavier (don't know if weight is an issue).

To keep it simple you can probably just use a single action cylinder (only provides power in one direction) which is returned to the original position by weight (and removing gas from the cylinder). You'll need a 3 way valve to pressurise and vent the cylinder at will. The piston would ideally be machined HDPE/delrin/nylon etc. however a slightly smaller PVC cap sanded to fit could work if you have limited tooling. Then a threaded rod to connect the piston and scissor lift.

An o-ring should definitely be used. If this is against the rules make your piston tight fitting (albeit with low friction) and grease it heavily. If using premade valves is against the rules you could use tubing as the valves (this would work OK with the low pressures you need): have two tubes going to the cylinder, one to your air source, one to the atmosphere. Both are kinked (kink could be held in place with a short section of small diameter pvc pipe). Unkink air hose to activate and lift, close air hose and unkink hose to the atmosphere to lower